144

ELEMENTS OF MATERIA MEDICA.

sea, as in the Himalayahs, cannot be derived from the atmosphere, not tomention the peculiar relations of the Icelandic Geysers” (Gairdner’sEssay on Mineral and Thermal Springs, p. 289). Other sources, there-fore, have been sought for, and the writer just quoted enumerates three;viz. the focus of volcanic activity, the great mass of the ocean, or othermasses of salt-water, and subterranean reservoirs.

Considered with reference to their temperature, mineral waters aredivided into cold and hot. The hot or thermal waters are those whichpossess a temperature more or less elevated above the mean of the lati-tude or elevation at which they are found, and the changes of which, ifany, observe no regular periods coincident with the revolutions of theseasons. Three causes have been assigned as the source of the heat of

mineral waters ; viz. volcanic action, now inexistence; volcanic action, now extinguished,but the effects of which still remain; and, acentral cause of heat, which increases aswe descend from the surface to the interiorof the earth (Gairdner, op. cit.).

The Geysers, or boiling springs, of Ice-land , are evidently connected with volcanicaction. They are intermittent fountains,which throw up boiling- water and sprayto a great height into the air. For furtherinformation concerning them, I must refer toSir G. S. Mackenzie’s “ Travels in Iceland during the Summer of 1810,” and to Barrow’s“ Visit to Iceland , by way of Tronyem,fyc., in the Summer of 1834.”

The origin of the saline and other constituents is another interestingtopic of inquiry connected with the natural history of mineral springs.As water in its passage through the different strata of the earth mustcome in contact with various substances which are soluble in it, we refercertain constituents of mineral waters to solution and lixiviation merely :as chloride of sodium, carbonates of lime and magnesia, iodides andbromides of sodium and magnesium, iron, silica, &c. Chemical ac-tion must, in some cases, be the source of other constituents. Thussulphuretted hydrogen is probably produced by the action of wateron some metallic sulphuret (especially .rronpyrites): sulphurous andsulphuric acid, from the oxidation and combustion of sulphur, free orcombined. The carbonic acid found in the acidulous or carbonatedwaters is referrible to the decomposition of carbonate of lime, either byheat or by the action of sulphuric acid. Hydrochloric acid is doubtlessproduced by the decomposition of some chloride or muriate (probablychloride of sodium or sal ammoniac). Carbonate of soda must also be con-sidered as the product of some chemical process ; thus, that found in thenatron lakes of Egypt is supposed to be formed by the action of chlorideof sodium on carbonate of lime (Bertholfett, Essai de Statique Chimique,l er . part. p. 406). “ The different orifices of the Karlsbad Spradel

discharge annually about 13,000 tons of carbonate of soda, and 20,000 ofthe sulphate in the crystallized state” (Gairdner, op. cit. p. 325): but a“ very simple calculation is sufficient to shew, that the Donnersberg alone, the loftiest of the Bohemian Mittelgebirge, a cone of clinkstone

Fig. 39.

New Geyser.

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